The invention relates to temperature-compensated scanning probe microscopes (SPMs), particularly "tube scanner" type SPMs.
The prior art includes scanning tunneling microscopes (STMs) of the type shown in FIGS. 1 and 2. FIG. 1 shows an atomic resolution scanning tunneling microscope of the type developed by Binnig and Rohrer. The design shown in FIG. 1 is referred to as a "tripod design". Tripod "scanning heads" are known to have problems with thermal drift and interaction between the x, y, and z piezoelectric transducers 13, 14, and 15 as shown in FIG. 1. Actuation of one of piezoelectric transducers 13, 14, or 15 that is intended to produce movement of probe 11 (and atom 12 on the lower tip of probe 11) along its corresponding x, y, or z axis inevitably produces some movement along the other two axes. In FIG. 1, numeral 16 designates an article to be scanned, and numeral 17 designates electrons on the surface of article 16 which may "tunnel" up to atom 12 when atom 12 gets within approximately a few Angstroms of the immediately underlying feature of sample 16. This produces a current I which is sensed by a current measuring device 19. Numeral 20 indicates a bias voltage applied to probe 11 to produce the tunneling effect.
"Tube scanner" STMs were developed by Binnig and Smith. In these devices, a single piezoelectric cylindrical tube, with its outer electrode divided into four equal quadrants parallel to the tube's cylindrical axis, provides lateral scanning motion at a free end of the tube (the other end of the tube being stationary), by bending when voltages are applied to adjacent outside quadrants. The prior tube scanner device also produces lateral displacement along the z axis when a common voltage is applied to all four quadrant conductors relative to the grounded inner electrode of the tube. Such tube scanner designs suffer from thermal drift along the z axis. In order to achieve a long scan range, such prior art tube scanners have a large axial dimension and a low mechanical resonance frequency, and hence are subject to mechanical vibrations which make it much more difficult to accurately move the probe tip 11 over the surface 17, because low frequency vibrations inherently are of larger amplitude, and also are more easily excited than high frequency vibrations. Larger amplitude vibrations make it much more difficult to maintain a constant distance between the probe tip and the surface being scanned. Such prior tube-type scanners also are undesirably large.
The several Angstrom distance of the probe tip to the sampled surface is so small that any low frequency mechanical vibration makes it very difficult for the electronic feedback loop to permit accurate scanning or tracking of the probe tip over the surface features. Any large scanning tube inherently has a lower mechanical resonance frequency and consequently is more problematic in this respect than a physically smaller scan tube. Large scanning tubes also inherently are much more sensitive to thermal variations, the magnitude of which are proportional to physical size.
The problem of thermal sensitivity is addressed in U.S. Pat. No. 4,841,148 (Lyding), which discloses an STM that is thermally compensated by providing a pair of concentric piezoelectric tubes of the same length and composition. A tunneling probe is attached to a free end of the inner tube, which is divided into equal lateral quadrants for providing transverse and axial scanning motion. In this device, shown in FIG. 2 hereof, the sample holder 32 rests on two spaced rods 31 attached to the outer end of tube 25. Support element 32 supports test surface 17. The dimensions and thermal expansion coefficient of inner tube 26 and outer tube 25 are identical, so as to compensate for thermal variations in the lengths of both tubes. The distance between the tip of probe 11 and the surface 17 of the sample 16 is relatively unaffected by temperature variations of the piezoelectric tubes 25 and 26 because temperature-caused expansion and contraction of the inner and outer tubes in the directions of arrows 33 and 33A are equal.
The Lyding device provides no solution to the above-mentioned problems of achieving a large scan range unless a large structure that has low mechanical resonance frequencies is used. Consequently, the Lyding device is relatively unsuitable for achieving a large scan range. An STM of the Lyding design would need to have its piezoelectric tubes approximately three to six inches long in order to achieve a scan range of roughly 100 microns. Furthermore, it is designed for horizontal mounting only, and is totally unsuitable for vertical mounting because the technique for supporting the sample holder relies on gravity to hold sample holder 32 on rails 31.
It would be desirable to be able to control scanning in the x, y, and z directions with separate, corresponding control voltages applied to separate electrodes on the piezoelectric tube, because if the control signals are combined, the motion of the free end of the tube in one direction due to one component of the combined control signal can offset the sensitivity of the piezoelectric device to another component of the combined control signal.
Abandoned patent application Ser. No. 305,637, by Elings et al., filed Feb. 3, 1989, and incorporated by reference in U.S. Pat. No. 5,051,646 issued Sep. 24, 1991 by the same inventors, entitled "SCANNER FOR A SCANNING PROBE MICROSCOPE", discloses four inner scan electrodes attached to the inner surface of an elongated cylindrical piezoelectric tube having a fixed end and a free end, and four outer scan electrodes attached to the outer surface and aligned, respectively, with the inner electrodes and application of separate x and y scan voltages to control scanning of the probe in the x and y directions. A cylindrical electrode is attached to an inner surface of another portion of the piezoelectric tube and outer cylindrical electrode attached to the outer surface thereof. Another voltage source controls scanning of the electrode in the z direction.
There is an unmet need for an improved compact scanning probe microscope which is thermally compensated, provides large axial and lateral motion of a tube type scanning probe along a test surface, and avoids low mechanical resonance frequencies which interfere with accurate profiling, and which allows separate voltages to control movement in the x, y, and z directions, respectively.